Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits electromagnetic radiation, typically light, in response to the application of an electrical potential difference across the electrodes. The electroluminescent medium must not only be capable of luminescing, but must also be capable of fabrication in a continuous form (i.e., must be pin hole free) and must be sufficiently stable to facilitate fabrication and to support device operation.
Initially organic EL devices were fabricated using single crystals of organic materials, as illustrated by Mehl et al U.S. Pat. No. 3,530,325 and Williams U.S. Pat. No. 3,621,321. Because single crystal organic electroluminescent layers were relatively difficult to fabricate and further did not readily lend themselves to thin layer constructions in thicknesses below about 50 .mu.m, the art turned to the use of thin film deposition techniques to form the organic layer of EL devices. Unfortunately, thin film deposition techniques produced devices which exhibited performance efficiencies 1 to 2 orders of magnitude below that obtained with single organic crystal devices.
In the last decade the art has developed a new class of organic EL devices hereinafter referred to as internal junction organic EL devices which lend themselves to thin film deposition techniques for fabrication of the organic layers and which exhibit performance characteristics comparable to or better than those of single organic crystal EL devices. This new class of organic EL devices has been made possible by dividing the organic medium separating the electrodes into a hole injecting and transporting zone and an electron injecting and transporting zone. The interface of the two organic zones constitute an internal junction allowing injection of holes into the electron injecting and transporting zone for recombination and luminescence, but blocking electron injection into the hole injecting and transporting zone. Examples of internal junction organic EL devices are provided by Tang U.S. Pat. No. 4,356,429, VanSlyke et al U.S. Pat. Nos. 4,539,507 and 4,720,432, and Tang et al U.S. Pat. No. 4,769,292.
Internal junction organic EL devices can be driven into luminescence using either an alternating current (AC) or direct current (DC) power source. Since luminescence occurs only when the electrode contacting the electron injecting and transporting zone is more negative than the electrode contacting the hole injecting and transporting zone (i.e., the device is forward biased), the former electrode is referred to as the device cathode while the latter electrode is referred to as the device anode.
While the art has encountered little difficulty in constructing fully acceptable stable anodes for internal junction organic EL devices, cathode construction has been a matter of extended investigation. In selecting a cathode metal, a balance must be struck between metals having the highest electron injecting efficiencies and those having the highest levels of stability. The highest electron injecting efficiencies are obtained with alkali metals, which are too unstable for convenient use, while metals having the highest stabilities show limited electron injection efficiencies and are, in fact, better suited for anode construction.
Tang U.S. Pat. No. 4,356,429 teaches to form cathodes of organic EL devices of metals such as indium, silver, tin, and aluminum. VanSlyke et al U.S. Pat. No. 4,539,507 teaches to form the cathodes of organic EL devices of metals such as silver, tin, lead, magnesium, manganese and aluminum.
Tang et al U.S. Pat. No. 4,885,211 found that a practical and efficient cathode for an internal junction organic EL device could be produced by employing at least 50 percent (atomic basis) magnesium in combination with at least 0.1 percent (atomic basis) of one other metal. Tang et al demonstrated that cathodes constructed entirely of magnesium were too unstable for practical use. Extended internal junction organic EL device operation was demonstrated by substituting for minor proportions of magnesium one or more of the metals silver, indium, tin, titanium, chromium, europium, antimony, tellurium, and manganese. In those instances in which the additional metal was a higher work function metal efficiencies were much higher than when the higher work function metals were used alone. The instability of magnesium only cathodes prevented establishing their efficiency level with certainty. When the proportion of the higher work function metal was increased above 50 percent (atomic basis), illustrated by a magnesium-silver concentration series, the initial and extended performance efficiencies of the cathode were significantly reduced. Apart from listing aluminum among known high (&gt;4.0 eV) work function metals, Tang et al contains no teaching relating specifically to the construction of aluminum containing electrodes.